The major goals of this study are: I. Independently force lung volume and CO2 concentration. II. Determine the effect of static and dynamically changing lung volume and CO2 concentration on the timing and amplitude of the ongoing breath. III. Differentiate between the effect of pulmonary receptors and other sensory afferents altering the output of the central respiratory neural oscillator. The Tegu lizard has been chosen because of the central respiratory neural oscillator. The Tegu lizard has been chosen because of the ease of independently manipulating pulmonary CO2 concentration and/or volume. Independence of CO2 concentration and pulmonary stretch is due to the simple sac-like nature of the Tegu's lungs. It can be continuously ventilated by blowing air into the trachea through the lungs and allowing the gas to exhaust by means of surgically implanted exit tubes. Using this preparation the levels of CO2 concentration and pulmonary stretch can be independently forced in either a dynamic or static fashion. By further blocking certain receptors it will be possible to evaluate the effect of the different sensory systems, such as pulmonary afferents or carotid chemoreceptors, on respiratory control. The hypothesis to be tested is that the CO2 concentration in the lung alters the timing of the ongoing breath. A better description of the way the central neural oscillator responds to peripheral inputs such as CO2 sensitive pulmonary mechanoreceptors is an important step in evaluating pulmonary function in humans. This study is particularly relevant in understanding the cause and thus possible treatment of respiratory disorders such as hyperventilation syndrome or sudden infant syndrome.